The invention relates to an acoustic transformer in the form of a waveguide with a generally circular entrance and rectangular exit, and which includes a fixed wall which, is in the center of the transformer, extends from one side to the other, divides the transformer into two parts of substantially equal size and divides the transformer into two waveguides of substantially identical shape and which have a cross sectional shape, which is initially in the shape of a semicircle and then changes over into the shape of a rectangle. The invention relates further to a method for transforming sound waves radiated by a circular or oval membrane into a rectangularly radiating surface.
Transforming is understood in the present context to be the conversion of a circular sound wave front into a broader rectangular sound wave of identical phase and amplitude, and a transformer is understood herein to be a waveguide which carries out this conversion.
For certain purposes, rectangular sound-radiating surfaces are required in electroacoustic sound radiation. Since electroacoustic generation of sound succeeds best with circular or oval membranes, an acoustic transformer, which previously was realized by different waveguides of different construction, is required to transform sound waves, which are radiated by a circular or oval membrane, into a rectangularly radiating surface.
Such an acoustic transformer is previously known from DE 689 15 582.4. A waveguide is described with a bulbous housing with a circular inlet for the sound waves and a rectangular outlet, in which there is a freely suspended body, which, on the inlet side, is formed as a cone having the width of the rectangular outlet slot and changes over on the outlet side into two flat surfaces, which extend at an angle to one another and to the rectangular exit. With the housing, enveloping this body, this waveguide, in the passing-through direction as well as in the circumferential direction, forms an uninterrupted passage for the sound waves, in which the latter pass over a path of equal length from the circular entrance to the rectangular exit. At the same time, the path for the sound waves initially forms an exact conical surface spreading the sound waves apart, and then an annular channel, which leads the sound waves together into a rectangular shape.
Measurements at the rectangular outlet of the above-described acoustic transformer have shown that the latter has resonances for certain frequencies, which is highly undesirable for the transmission of sound and music performances.
The horns of musical instruments and automobiles are other types of acoustic transformers. These also have a curved sound path, which expands in diameter and sometimes has an oval exit, but do not have any sound-dividing internal parts, nor the sound-radiating rectangular surface at the exit. They serve to amplify and concentrate the sound.
A special shape of horn loudspeaker is also disclosed in U.S. Pat. No. 4,091,891, having a partition which is disposed centrally in the horn and terminates in material thickenings at its sides. By these means, two mutually adjacent horns are created, which are to bring about an improved funnel-shaped radiation of low, as well as middle, frequencies.
An object of the invention is therefore to avoid the disadvantages of the prior art.
It is a further object of the invention to provide an acoustic transformer without pronounced resonances, which undertakes the transformation over the whole of the exit in phase and at the same amplitude.